Cement kilns are used to manufacture of Portland and other types of hydraulic cement by a process often called pyroprocessing. In the process, calcium carbonate is reacted with silica-bearing minerals and forms a variety of mixed calcium silicates. These kilns consume large amounts of fuel and create large amounts of combustion gases in need of treatment.
Cement plants can be divided into two major categories, wet and dry, while there are also variations of these. In the dry kiln process, raw materials are first milled and then fed to the kiln. In wet-process plants, water is added as a part of the process, typically to a ball mill or the like for grinding the raw materials to produce a slurry to feed the kiln. A hybrid, or semidry process, mixes water with a dry raw mix in a pelletizer to form pellets that are conveyed through a preheater, where they are dried and partially calcined before entering a rotary kiln.
Depending on the process, the desired form of feed mixture will be fed into an elevated end of a rotating kiln to be processed and discharged from a low end. Combustion of fuel supplies heat for the process from a burner located at the low end. The fuel can be any carbonaceous fuel, but is typically coal and in some cases can include refuse.
The kilns are operated at high temperatures and the fuel materials are essentially completely combusted. Solid residue from the combustion will be incorporated into cement clinker that is formed. Gaseous combustion products pass upward through the kiln and a series of cyclones that separate out entrained solids and can pretreat incoming feed. Temperatures in the kiln are high, typically on the order of 3,000° F. or so.
The raw materials and fuel will produce gaseous combustion products that include chlorine or sulfur. Combustion of these materials can form gaseous HCl, as well as mercury and SOx, in the combustion gases. Combustion can also form low-melting compounds that can form scale on internal walls of process equipment. The scale can cause processing problems including increased pressures in the gasses and partial or complete clogging in one or more cyclones.
The art has developed a number of measures to lessen these problems, but all have expenses and limitations. The formation of scale deposits corresponds to the melting points of compounds formed and the temperature distribution in ducts, cyclones and other equipment used at the gas discharge end. Scale formation at these locations has been mainly attributed to sulfides and chlorides. In one proposed solution, chlorine bypass technology proposes recovering chlorine from where chlorine is condensed most, e.g., a preheater section. Chlorine recovery can reduce chlorine from condensing on raw materials and can help mitigate the formation of scale caused by chlorine condensation.
In other technologies, the scale is permitted to form and then addressed with a descaler such as an air blaster, which descales by blowing compressed air, or a soot blower, which can periodically blow high-pressure steam or water where the scale forms.
The release of mercury emissions from cement kilns is largely a result of the type of fuel used, but can also be caused by cement feed materials. Many techniques have been tried, but they all have high costs and limitations.
The problems with emissions of hydrochloric acid and mercury are particularly troublesome and have not been adequately addressed by the many technologies used to reduce them. Moreover, known technologies, such as adsorption on carbon, can adversely affect the quality of the cement produced.
It would be important to the environment if hydrochloric acid, desirably along with SOx and/or mercury, could be well and economically controlled in the production of cement, preferably with minimal negative effect on the cement and with improved economies of operation such as by controlling scale formation during processing.